New here, but somewhat in the same boat.
I understand your dilemma. I was just accepted to Columbia at a master's level, rather than the PhD I applied for: same department, different track. However, their faculty for my interest is second to none. Though I have not received a firm word on a PhD acceptance from others, yet, I am seriously thinking, who cares? This is a rather large foot in the door to the best department for my focus, why not do another 30 credit masters and then the PhD. I assume this what you're thinking with the Stanford POLS masters.
I'm not too excited about the POLS possibility, myself. A nine month masters w/out funding seems kind of silly for me. However, if that is where you would like to end up for your eventual PhD, I can see that being a viable option/tryout.
*Not trying to take your USC spot.